Bug#12704861

As Mark pointed out, there isn’t a lot of detail in the release notes about what could potentially be a very serious problem that is fixed in MySQL 5.1.60. I’ll repeat here the full documentation from the release notes:

“InnoDB Storage Engine: Data from BLOB columns could be lost if the server crashed at a precise moment when other columns were being updated in an InnoDB table. (Bug #12704861)”

This wasn’t much detail…so I went digging. Even though the BZR trees have not yet been updated with the 5.1.60 release tags, there’s enough history there to attempt to piece together what the story is.

Sometime after 5.1.57, Marko fixed “Bug#12612184 Race condition after btr_cur_pessimistic_update()” (revision id: marko.makela@oracle.com-20110616072721-8bo92ctixq6eqavr ). I mention this bug as it is mentioned in the commit message for the fix for the bug we’re interested in, 12704861.

The fix for our bug is in revision id marko.makela@oracle.com-20110829081642-z0w992a0mrc62s6w (so you can find it in the bzr tree if you’re so interested). The first paragraph of the commit message is rather telling:

The fix of Bug#12612184 broke crash recovery. When a record that contains off-page columns (BLOBs) is updated, we must first write redo log about the BLOB page writes, and only after that write the redo log about the B-tree changes. The buggy fix would log the B-tree changes first, meaning that after recovery, we could end up having a record that contains a null BLOB pointer.

So, from this we can tell that crash recovery for updated records that contain BLOBs is broken in 5.1.58 and 5.1.59. Now, just because this bug is there doesn’t mean you’ll hit it all the time, InnoDB has managed to have recovery bugs for a very long time before and people only seldom hit them.

The last paragraph of the commit message is also interesting:

There is a known issue in tablespace extension. If the request to allocate a BLOB page leads to the tablespace being extended, crash recovery could see BLOB writes to pages that are off the tablespace file bounds. This should trigger an assertion failure in fil_io() at crash recovery. The safe thing would be to write redo log about the tablespace extension to the mini-transaction of the BLOB write, not to the mini-transaction of the record update. However, there is no redo log record for file extension in the current redo log format.

What does this mean? If writing a BLOB causes the tablespace to be extended and you crash at exactly the right time you can end up with a situation where crash recovery will itself crash. The last sentence (“there is no redo log record for file extension in the current redo log format”) to me suggests that if we were to go looking for bugs, we could certainly find more around various operations and growing tablespace files. The two obvious recommendations to help avoid hitting such bugs: don’t use autoextend and create huge initial tablespaces both have drawbacks. Perhaps a compromise is to have the autoextend increment be sufficiently large that you won’t do it too often and greatly reduce your exposure to such bugs.

For completeness, I’ll include the full text of Marko’s commit message for the fix as it’s interesting reading for those of us who are into InnoDB internals:

The fix of Bug#12612184 broke crash recovery. When a record that contains off-page columns (BLOBs) is updated, we must first write redo log about the BLOB page writes, and only after that write the redo log about the B-tree changes. The buggy fix would log the B-tree changes first, meaning that after recovery, we could end up having a record that contains a null BLOB pointer.

Because we will be redo logging the writes off the off-page columns before the B-tree changes, we must make sure that the pages chosen for the off-page columns are free both before and after the B-tree changes. In this way, the worst thing that can happen in crash recovery is that the BLOBs are written to free pages, but the B-tree changes are not applied. The BLOB pages would correctly remain free in this case. To achieve this, we must allocate the BLOB pages in the mini-transaction of the B-tree operation. A further quirk is that BLOB pages are allocated from the same file segment as leaf pages. Because of this, we must temporarily “hide” any leaf pages that were freed during the B-tree operation by “fake allocating” them prior to writing the BLOBs, and freeing them again before the mtr_commit() of the B-tree operation, in btr_mark_freed_leaves().

btr_cur_mtr_commit_and_start(): Remove this faulty function that was introduced in the Bug#12612184 fix. The problem that this function was trying to address was that when we did mtr_commit() the BLOB writes before the mtr_commit() of the update, the new BLOB pages could have overwritten clustered index B-tree leaf pages that were freed during the update. If recovery applied the redo log of the BLOB writes but did not see the log of the record update, the index tree would be corrupted. The correct solution is to make the freed clustered index pages unavailable to the BLOB allocation. This function is also a likely culprit of InnoDB hangs that were observed when testing the Bug#12612184 fix.

btr_mark_freed_leaves(): Mark all freed clustered index leaf pages of a mini-transaction allocated (nonfree=TRUE) before storing the BLOBs, or freed (nonfree=FALSE) before committing the mini-transaction.

btr_freed_leaves_validate(): A debug function for checking that all clustered index leaf pages that have been marked free in the mini-transaction are consistent (have not been zeroed out).

btr_page_alloc_low(): Refactored from btr_page_alloc(). Return the number of the allocated page, or FIL_NULL if out of space. Add the parameter “mtr_t* init_mtr” for specifying the mini-transaction where the page should be initialized, or if this is a “fake allocation” (init_mtr=NULL) by btr_mark_freed_leaves(nonfree=TRUE).

btr_page_alloc(): Add the parameter init_mtr, allowing the page to be initialized and X-latched in a different mini-transaction than the one that is used for the allocation. Invoke btr_page_alloc_low(). If a clustered index leaf page was previously freed in mtr, remove it from the memo of previously freed pages.

btr_page_free(): Assert that the page is a B-tree page and it has been X-latched by the mini-transaction. If the freed page was a leaf page of a clustered index, link it by a MTR_MEMO_FREE_CLUST_LEAF marker to the mini-transaction.

btr_store_big_rec_extern_fields_func(): Add the parameter alloc_mtr, which is NULL (old behaviour in inserts) and the same as local_mtr in updates. If alloc_mtr!=NULL, the BLOB pages will be allocated from it instead of the mini-transaction that is used for writing the BLOBs.

fsp_alloc_from_free_frag(): Refactored from fsp_alloc_free_page(). Allocate the specified page from a partially free extent.

fseg_alloc_free_page_low(), fseg_alloc_free_page_general(): Add the parameter “mtr_t* init_mtr” for specifying the mini-transaction where the page should be initialized, or NULL if this is a “fake allocation” that prevents the reuse of a previously freed B-tree page for BLOB storage. If init_mtr==NULL, try harder to reallocate the specified page and assert that it succeeded.

fsp_alloc_free_page(): Add the parameter “mtr_t* init_mtr” for specifying the mini-transaction where the page should be initialized. Do not allow init_mtr == NULL, because this function is never to be used for “fake allocations”.

mtr_t: Add the operation MTR_MEMO_FREE_CLUST_LEAF and the flag mtr->freed_clust_leaf for quickly determining if any MTR_MEMO_FREE_CLUST_LEAF operations have been posted.

row_ins_index_entry_low(): When columns are being made off-page in insert-by-update, invoke btr_mark_freed_leaves(nonfree=TRUE) and pass the mini-transaction as the alloc_mtr to btr_store_big_rec_extern_fields(). Finally, invoke btr_mark_freed_leaves(nonfree=FALSE) to avoid leaking pages.

row_build(): Correct a comment, and add a debug assertion that a record that contains NULL BLOB pointers must be a fresh insert.

row_upd_clust_rec(): When columns are being moved off-page, invoke btr_mark_freed_leaves(nonfree=TRUE) and pass the mini-transaction as the alloc_mtr to btr_store_big_rec_extern_fields(). Finally, invoke btr_mark_freed_leaves(nonfree=FALSE) to avoid leaking pages.

There is a known issue in tablespace extension. If the request to allocate a BLOB page leads to the tablespace being extended, crash recovery could see BLOB writes to pages that are off the tablespace file bounds. This should trigger an assertion failure in fil_io() at crash recovery. The safe thing would be to write redo log about the tablespace extension to the mini-transaction of the BLOB write, not to the mini-transaction of the record update. However, there is no redo log record for file extension in the current redo log format.

About Stewart Smith

Stewart Smith has a deep background in database internals including MySQL, MySQL Cluster, Drizzle, InnoDB and HailDB. he is also one of the founding core developers of the Drizzle database server. He served at Percona from 2011-2014. He is a former Percona employee.